The present invention relates to aqueous self-crosslinking PU dispersions, to stoving enamels prepared from them, and to the use thereof in varnishes and paints, especially in automotive OEM coatings.
Recent years have seen an increase in the importance of aqueous paints and coating compositions, due to ever more stringent emissions guidelines in terms of the solvents released on application of the coating materials. Although aqueous coating systems are now available for many areas of application, they are often unable to match the high quality level of conventional, solvent-borne paints in terms of solvent resistance and chemical resistance or else in terms of elasticity and mechanical durability. In particular, to date, there have been no disclosures of polyurethane-based coating compositions which can be processed from the aqueous phase and which adequately meet the exacting requirements of the industry for automotive OEM finishing.
This is true not only of DE-A 4,001,783, which describes anionically modified aliphatic polyisocyanates, but also of the systems of DE-A 2,456,469, DE-A 2,814,815, EP-A 0 012,348 and EP-A 0 424,697, which describe aqueous stoving enamel binders based on blocked polyisocyanates and organic polyhydroxyl compounds. Additionally, the systems based on carboxyl-containing polyurethane prepolymers with blocked isocyanate groups, in accordance with DE-A 2,708,611, or the blocked water-soluble urethane prepolymers of DE-A 3,234,590, which are of high functionality and therefore largely unsuited to the production of elastic coatings, are also substantially unusable for this specified purpose.
Further improvements have been attained in recent years in the one-component (1K) stoving enamels used, such as, for example, in EP-A 0 576,952, which describes combinations of water-soluble or water-dispersible polyhydroxy compounds with water-soluble or dispersible blocked polyisocyanates, or in DE-A 19,930,555, which discloses combinations of a water-dispersible, hydroxy-functional binder component containing urethane groups, a binder component containing blocked isocyanate groups (which is prepared in a multi-stage reaction by way of two prepolymerization steps), an amino resin and further components. A disadvantage of these one-component systems is that the prepared components are then formulated to give coating materials, and so necessitate an additional mixing step.
The coating materials described in the prior art do not, however, meet all of the requirements on the part of the industry, including those relating to the solids content of varnishes and paints produced from them as well as their stability, as well as surface smoothness and gloss of coatings made of (?) them.
An object of the present invention was to provide improved 1K baking systems, where the coating materials should in particular have a high solids content and that the coatings should have a high gloss.